The invention relates to an optical scanning device for optically scanning a record carrier an information plane and a transparent layer, the device comprising an objective lens and a plano-convex lens for converging a radiation beam through the transparent layer to a focus on the information layer, the plano-convex lens having a convex surface facing the objective lens, and a planar surface facing the transparent layer.
The amount of information that can be stored on an optical record carrier depends inter alia on the size of the radiation spot formed by the scanning device on the information layer of the record carrier. The information density and hence the amount of stored information can be increased by decreasing the size of the spot. The spot size can be reduced by increasing the numerical aperture of the radiation beam forming the spot. When using a single objective lens, such an increase of the numerical aperture is in general accompanied by a decrease of the free working distance of the lens forming the radiation beam, i.e. the smallest distance between the record and the lens. At higher numerical apertures, the manufacturing costs of such objective lenses become high, the field of the lens reduces and the dispersion of the material the lens is made of gives increasing problems. The problems may be mitigated by inserting a plano-convex lens between the objective lens and the record carrier. The plano-convex lens, sometimes called a slider lens or a solid immersion lens, is arranged at a very small distance above the record carrier. The convergence of the radiation beam is then distributed over the objective lens and the plano-convex lens. An advantage of the use of the plano-convex lens is that it hardly adds aberrations to the radiation beam.
A scanning device having such a plano-convex lens is known from the European patent application no. 0 727 777. The device includes an optical head in which an objective lens and a plano-convex lens converge a radiation beam to a numerical aperture (NA) of 0.84 for scanning the record carrier. The plano-convex lens is arranged at a small height above the record carrier. The lens may be mounted on a slider in sliding contact with the record carrier or floating on a thin air layer. The lens has a free working distance of several micrometers. When the optical head of the device, flying above the surface of the record carrier, collides with a dust particle on the surface, the head and record carrier will. be damaged. The free working distance should therefore be larger than the size of the contamination expected on the record carrier. A disadvantage of the known device is that the manufacture of the optical head of the device becomes increasingly difficult at increasing free working distance.